A hundred years ago, the documentation of the Latvian dialects in earnest picked up steam with the foundation of the philological society (Filologu biedriba) in Riga, which in several ways marked the beginning of the golden era of Latvian dialectology. Since then, progress has been steady on the descriptive front, but in order to get adiachronic perspective, one was for a long time referred to Endzelins' historical grammar of Latvian from 1922. It is only in recent decades that the conclusions in this monumental work have begun to be challenged, and although its importance can hardly be overemphasised, the need for a new theoretical framework is obvious. This study is an attempt to modernise the approach to Latvian historical linguistics. Specifically, it aims to increase the contribution of the dialects by operating with the comparative evidence of all the dialectal material that is available today. Apart from a general investigation of shortening and apocope, the thesis contains exhaustive dialectological studies of the present 2nd singular, active preterite participles to preterite stems in j and the locative singular endings. By offering diachronic analyses of the full dialectal testimony to the investigated categories, the thesis serves as a step towards a more detailed picture of the prehistory of Latvian and a deeper understanding of the processes that led to the attested linguistic stage. This is a Doctoral Thesis in Baltic Languages at Stockholm University, Sweden 2020